This invention relates generally to a heat and pressure fuser for an electrophotographic printing machine, and more particularly the invention is directed to a Nip Forming Fuser Roll (NFFR) structure including an external heat source and shield for preventing misstripped substrates from contacting the external heat source.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent image on the photoconductive member. The toner attracted to a donor roll is then deposited on a latent electrostatic images on a charge retentive surface which is usually a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser (NFFR), the heated fuser roll is provided with a layer or layers that are deformable by a harder pressure roll when the two rolls are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll.
The heated fuser roll is usually the roll that contacts the toner images on a substrate such as plain paper. In any event, the roll contacting the toner images is fabricated with a relatively thick, readily deformable, elastomeric layer of layers for providing a roll structure that is self-stripping.
With the requirement for increased productivity or the ever increasing speeds at which consumers want their copies produced, come the ever increasing problems associated with fusing such images at the higher speeds. One type of fuser that allows for higher speeds utilizes an external thermal energy source which cooperates with an internal energy source for effecting high throughput speed. While the heated fuser roll of such a fuser is fabricated such that it is self-stripping misstrips occasionally occur. When a misstrip does occur it is necessary to prevent contact of the external heat source by the substrate carrying the toner images.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,268,559 granted to Robert M. Jacobs on Dec. 7, 1993 discloses a belt fuser which is self-stripping. The belt and a pressure roll form a nip through substrates carrying toner images pass with the toner images contacting the belt. The belt is entrained about a plurality of rollers for movement in an endless path. One of the rollers which is a drive roller for effecting belt movement is overdriven for causing a post-nip extent of the belt to stretch for effecting separation of the substrates from the belt.
U.S. Pat. No. 4,114,021 granted to Nishide et al on Sep. 12, 1978 discloses a heated roll fixing device for electrophotographic copying machines, wherein a first roll is coated on the surface with a heat-resistant releasing agent and is adapted to contact toner images on a supporting body. A second roll cooperates with the first roll for pressure and heat fixing the toner images to the supporting body. The fixing device is characterized, in that, either the first or second roll but at least the first roll is provided with an inner heating means and an outer heating means, the latter heating means having a small heat capacity which is constructed in a manner to surround the first roll.
U.S. Pat. No. 4,021,641 granted to John F. Elter on May 3, 1977 relates to an apparatus for heat fixing toner images electrostatically adhered to copy paper. The apparatus is characterized by the provision of a radiant energy source together with a parabolic reflector and lens structure for focusing the energy onto the toner images. The lens structure in the preferred embodiment comprises a Fresnel lens which is fabricated in the form of a belt entrained about the reflector and energy source which belt is adapted to be moved relative to the reflector and the heat source.
U.S. Pat. No. 3,898,424 granted to Raghulinga R. Thettu on Aug. 5, 1975 relates to an apparatus for heat fixing toner images electrostatically adhered to copy paper. The apparatus is characterized by the provision of plural radiant energy sources capable of fusing low density as well as high density images in an efficient manner. In order to prevent physical contact of the radiant energy sources by the copy paper, a shield is provided which is transparent to energy in the wave length bands required for fusing high and low density images.